Conventional water-removable ink was the one using supersaturated ink filtered in the manufacture of ink. Therefore, the supply of ink on a writing material such as paper or white board from a writing tool such as pen was not always controlled constant. At an early stage of using a writing pen, an excessive supply of ink produced unintended contamination around written notes, and after the lapse of some time, a nozzle of the pen might be blocked, which deteriorated the freshness of the ink color. Also, after the written notes have been erased, yellow trace was left.
In this regard, Japanese Patent Publication No. 1997-59547 by Kawashima Kiyoharu disclosed “colored ink and writing system using the same”, which comprises a developer including more than one electron accepting compound selected from adipic acid, ascorbic acid, zinc chloride, calcium chloride, etc.; a coupler including more than one electron donating compound selected from 3-diethylamino-7-chlorofluoran, rhodamine B lactam, etc.; a coupling maintaining agent such as polyvinyl alcohol, arginic acid, and cellulose; and a surfactant. This patent publication provided a new ink composition, which could be erased by water only and be compared to a conventional technology wherein written notes had been erased using solvents. The ink composition in this patent publication used leuco dyes and the developer, and it was convenient and simple material to erase.
However, although the written notes using the above ink composition could be erased by water, since the hydrophilic property of ink itself was low, it was difficult to erase the written notes clearly by using small amount of water. Also, in erasing the written notes, the ink was drifted, and after erasing, residuum of the ink left, which caused yellowing phenomenon on the writing material such as white board or paper. Further, as the fluidity of the ink was deteriorated, it was difficult to expect uniform distribution of color. Furthermore, compared to conventional oily and aqueous pens, chromaticness of the color of the ink deteriorated.
Meanwhile, conventional writing pens using colored ink mostly had advantages in that an inherent color of the ink was released immediately upon writing.
However, if such revelation of color of the ink is delayed for the time being, it will not only be able to stimulate the curiosity of infants or juveniles, thus enhancing learning effect, but also to satisfy various desires of students or adults in terms of ornamental purposes. Up to now, however, writing tools using such ink with an effect of coupling retardation have not yet been developed.
In this regard, the applicant of the present invention disclosed “water-removable hydrated writing material” in Korean Patent Publication No. 2002-6818, which comprises an electron-donating coloring agent including 3,3-bis (4-dimethylaminophenyl)-6-dimethylaminophthalide, etc.; an electron-accepting developer including citric acid, gallic acid, succinic acid, etc., with the coloring agent and the developer being dissolved in an organic solvent; a binder; a surfactant; and an anti-oxidizing agent. According to the patent publication, the writing material showed a rapid fading-away effect of color by water, which changes little after a lapse of some time. Also, it was not harmful to the human body, but was environment-friendly.
However, in practice, the rapid fading-away effect of the above writing material fell short of our expectation. Further, it has never been expected and intended that the above writing material would show an effect of coupling retardation.